Various applications, such as robotics, require extremely precise positioning of mechanical elements, such as robot arms. Typically, such precise positioning is achieved by employing an electric motor, a reducing gear transmission, such as a harmonic drive gear, a shaft encoder and a tachometer. Both the tachometer and the shaft encoder provide output data to servo mechanisms which in turn provide position correction signals to the motor.
Normally the shaft encoder and the tachometer are directly mounted on the electric motor, downstream of the transmission. It may be appreciated that in this configuration, whatever the accuracy of the shaft encoder and the electric motor, such is not the accuracy of the positioning of the element driven thereby due to inaccuracies introduced by the transmission, as well as inaccuracies due to flexing of driven elements downstream of the transmission.
Positioning accuracies realized by conventional positioning devices of the type described hereinabove are of the order of milliradians to hundreds of microradians for rotary applications and of the order of hundredths or tenths of millimeters for linear applications.
Some of the inaccuracies associated with the transmission itself could be overcome by mounting a shaft encoder at the output of the gear, which is directly coupled to the driven element. This, however is impractical in most robotics applications due to space and weight limitations, complications in mounting, limitations on the movement of the robot, and cost.
A closed loop torque sensing and control system is described in "Joint Torque Control by a Direct Feedback for Industrial Robots" by J. Y. S. Luh, W. D. Fisher and Richard P. C. Paul, IEEE Transactions on Automatic Control, Vol. AC-28 No. 2 February 1983, pp. 153-160.
Compensation for bending of a load arm is described in "A Robot-Arm with Compensation for Bending" by P. C. Mulders et al, Annals of the CIRP, 35/1/1986 pp. 305-308.
The current state of the art is described in M. Vidyasagar, "System Theory and Robotics", IEEE Control Systems Magazine, Vol 7, No. 2, April, 1987, pp 16 and 17.